solving for Diffusivity

I am using the Transport of Dilute Species Module(time dependent study) for my project. In this module I am required to solve for N according to the equation N=-D∇c where D is the diffusivity and c is the concentration . Is there a way to solve for Diffusivity (D) if the other two parameters are known? Or is there another way to solve for it? , as the module will only let me solve for N.

Thank you